1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device, and more particularly, to an ESD protection device having improved ESD characteristics and reliability that includes discharge electrodes disposed in a cavity of an insulating substrate and arranged to face each other.
2. Description of the Related Art
ESD is a phenomenon in which strong electrical discharge is generated when a charged conductive body (e.g., human body) comes into contact with or comes sufficiently close to another conductive body (e.g., an electronic device). ESD causes damage or malfunctioning of electronic devices. To prevent this, it is necessary to prevent an excessively high voltage generated during discharge from being transmitted to circuits of electronic devices. ESD protection devices, which are also called surge absorbers, are used for such an application.
An ESD protection device is disposed, for instance, between a signal line and a ground of the circuit. The ESD protection device includes a pair of discharge electrodes facing each other with a space provided therebetween. Therefore, the ESD protection device has high resistance under normal operation and a signal is not sent to the ground. An excessively high voltage, for example, generated by static electricity through an antenna of a mobile phone causes discharge between the discharge electrodes of the ESD protection device, which directs the static electricity to the ground. Thus, a voltage generated by static electricity is not applied to the circuits disposed downstream from the ESD protection device, which protects the circuits from the static electricity.
For example, an ESD protection device shown in an exploded perspective view of FIG. 11 and a sectional view of FIG. 12 includes a cavity 5 provided in a ceramic multilayer substrate 7 including a plurality of laminated insulating ceramic sheets 2. Discharge electrodes 6 facing each other and electrically connected to external electrodes 1 are disposed in the cavity 5 which contains discharge gas. When a breakdown voltage is applied between the discharge electrodes 6, discharge is generated between the discharge electrodes 6 in the cavity 5, which directs an excessive voltage to the ground. Consequently, the circuits disposed downstream from the ESD protection device are protected (e.g., refer to Japanese Unexamined Patent Application Publication No. 2001-43954).
However, in such an ESD protection device, the responsivity to ESD varies due to variations in the space between the discharge electrodes. Furthermore, although the responsivity to ESD needs to be adjusted using an area of the region sandwiched between discharge electrodes facing each other, the amount of adjustment is limited by the size of a product or other factors. Therefore, it is often difficult to achieve desired responsivity to ESD.
Thus, the discharge phenomenon may be efficiently generated by a structure in which a conductive material is dispersed between discharge electrodes as in a Comparative Example described later. However, in such a structure, the conductive material is scattered due to the shock during discharge and, thus, the distribution density is decreased. This gradually increases discharge voltage after every discharge, and the discharge characteristics are degraded because of the repetitive discharges.